What's the point? Why argue over religion? What's it solving?

Why the constant argument between who's right and who's wrong? What happened to peace, and allowing others to believe what they want to? Why can't I say "I'm a Christian" or say "I'm not a Christian" without receiving some sort of slander from some social or religious group? What happened to just leaving people alone? If you're accusing someone of having a smaller brain because they believe in something you don't, where's your tolerance?